THE ARCHiSAN ROCKS. 501 



routh end, the gneiss resembles that at the northern end, and contams large masses of 

 white quartz, up to ten by four feet in size. 



These taiotty gneisses resemble closely others wliich occur on the hue of the Wiscon- 

 sin Valley Railroad, in Portage coanty. 



Three-quarters of a mUe west of NefllsvUle, at the crossing of Black river, on the S. 

 W. qr. Sec. 15, T. 24, R. 2 W., fine-grained, light-pinkish, sUghtly gneissoid, and very 

 quartzose granite (983) is exposed, with a vertical dip and E. W. strike. This rock is 

 very hard and compact, and appears to be a fine ornamental granite. 



Tne gneissoid and red granites of Black and Yellow rivers resemble one another 

 closely, and appear to be directly continuous with one another underneath the sandstone, 

 which nearly everywhere between the two rivers is the surface rock. Occasionally the 

 crystaUine rooks come to the sm-face in the interval, and are then of the same character 

 as on tlie rivers; as for mstance, on O'Neil's creek, in Sees. 1 and 2, T. 24 R. 1 W., 

 Clark county, where red granite is exposed; and on a high bluff in tlie N. E. part of T. 

 23. R. 2 E., whose upper portions are reported to be of red granite with sandstone layers 

 at lower levels. 



The amount of these reddish ornamental granites of extraordinarily fine quality occur- 

 ring on Yellow and Black rivers, and in the intervening country, appears to be very 

 great. 



THE ISOLATED ARCH^AN AREAS. 



I. In General. 



We have next to consider those isolated areas of Archaean rocks 

 which are found protruding through the surrounding horizontal Silu- 

 rian strata, at points widely scattered over the central part of the 

 state. Plate XYIII is a sketch map showing the relative positions 

 of the-se various areas, as also the nature of the rocks of each area, 

 and its distance from the southern boundary of the main Archaean 

 mass. Near to this boundary line it is not always possible to be sure 

 that we have to deal with an isolated area, when we find a mound-like 

 exposure of crystalline rocks, with sandstone showing in the vicinity 

 at lower levels, on account of the intricate and somewhat indefinite 

 nature of the boundary itself. Besides these doubtful areas, which 

 may be somewhere connected with the main Archaean region without 

 intervening horizontal strata, there are, however, many others which 

 occur as much as 50 or 100 miles within the region of the Lower Si- 

 lurian rocks. Underneath these, the connection with the rocks of the 

 main Archaean area is, of course, preserved, the separation being su- 

 perficial only. All of the scattered patches are but points of the uni- 

 versal Archaean basement, upon which all the later strata are built, 

 having earned their especial immunity from complete burial by virtuo 

 of the resistant nature of their materials. They are properly buried 

 mountains, and were high islands and reef -ledges in the early Paleozoic 

 yeas. 



